The combustion of fossil fuels unavoidably leads to the formation of nitrogen oxides (NOx) which, if released to the atmosphere, can lead to acid rain and detrimental interactions with ozone. Increasing government regulations around the world have called for continued NOx emission reductions from both stationary and mobile sources, requiring new and improved technologies that can remove NOx from exhaust streams. While three-way catalysts have been optimized to reduce NOx emissions in rich-burn exhaust as from consumer automobiles, these catalysts are ineffective under lean-burn conditions, as usually observed with stationary sources or diesel engines. Reduction of NOx with ammonia has been used industrially for stationary sources. However, it is less desirable (especially in transportation systems) due to the toxic nature of the ammonia which must be stored locally. Alternative catalytic processes have been investigated for operation under lean burn conditions, with the selective catalytic reduction (SCR) of NOx with hydrocarbons being one of the most promising. These hydrocarbons are more readily available and pose less of a emission problem.
A large number of materials have been found to be catalyticly active for SCR. By far, the most widely reported in the open literature are metal-exchanged zeolites, such as Cu-ZSM-5, Co-ZSM-5, and Fe-ZSM-5. These have been found to be very active for SCR using C3 hydrocarbons, with Cu-ZSM-5 being the most active. However, the zeolite-based materials lose much of their activity when water (common in exhaust streams) is added to the exhaust stream. The exact effects of water are not well known; it is speculated that either dealumination of the zeolite framework occurs which reduces the number of active sites, or that the metal sites agglomerate or are over-oxidized and lose their activity. While other catalysts that possess greater water stability have been found, such as metal-supported oxides, these tend to lack the desired activity seen by the metal zeolites, and produce large amounts of N2O (also a major pollutant).
Patents which have varying relationships to the inventions at issue are, for instance, U.S. Pat. No. 5,116,586 issued to Baacke et al., May 26, 1992. This patent relates to a zeolite catalyst and a method for reduction of nitrogen oxides by mixing the waste gas with ammonia. As previously discussed in the background of this invention, the use of ammonia in the reduction of nitrogen oxide requires entirely different conditions than the present invention which is an NOx reduction catalyst utilizing hydrocarbons. The Baacke et al. patent does not show or suggest a two phase catalyst or the method of making the inventive two phase catalyst, nor obviously, the present catalysts themselves in which the stabilizing oxide coats the molecular sieve or zeolite material. Moreover, the use of ammonia itself presents a hazard.
The Green et al. U.S. Pat. No. 4,962,075 issued Oct. 9, 1990 discloses a single phase catalyst obtained by intimately mixing the powders of the oxide binder and the metal salts so that although a physical mixture is formed, the inventive two phase catalyst is not made. This patent teaches mixing particles of a metal exchanged zeolite with an oxide. In this case, both particles are usually quite large (500 nm to 50,000 nm) and although the particles physically contact each other, no two phase catalyst with intimate contact between the phases such as produced by the methods of Examples 1-3 hereafter.
The Green et al. U.S. Pat. No. 5,276,249 issued Jan. 4, 1994 relates to a catalyst composition for the destruction of halogenated organics. The Green et al. '249 patent like the Green et al. '075 patent does not show or suggest the inventive method of making the two phase catalyst of the present invention, and therefore, does not show or suggest the present inventive catalyst. Although the '075 patent does disclose the use of cerium oxide, the Green et al. '249 patent does not, neither patent teaches the present invention which uses metal oxide sols for creating the second phase.
Also cited in the parent case is the Sachtler et al. U.S. Pat. No. 6,143,681 issued Nov. 7, 2000. The Sachtler et al. patent uses an entirely different means of introducing the metal into the zeolite, such as by metal vapor deposition, which leaves the metal in a different state and the addition of cerium into the matrix is by ion exchange not by a sol as taught in the present invention. Moreover, the '681 patent does not show or suggest the addition of a rare earth oxide by the use of a sol as opposed to a salt. Therefore, the '681 patent as the other patents cited in the prior case do not show the method of the present invention and therefore do not show the two phase catalyst of the present invention.